1. Field of the Invention
This invention relates to artillery shells and more particularly to an apparatus and method for reducing drag on a gun-launched artillery shell.
2. Description of the Related Art
For reasons concerning firing technology, artillery shells have a rear surface at right angles to the shell axis. It is well known that a rear surface that does not taper, or tapers too quickly, will cause the airflow to separate from the projectile at that location resulting in low pressure behind the shell. The low-pressure region acts like a partial vacuum over the entire aft area of the shell, which increases drag thus limiting the maximum range of the shell. The larger the area that the low pressure acts upon the greater the applied drag force.
The “base-bleed” technique has been much used in recent years to increase the range of air-defense and artillery shells without having to increase muzzle velocity and thereby increase the size of the propellant charge to a level the gun in question would not withstand. The base-bleed technique allows gas to flow out from the rear surface of the shell preferably at a flow rate that re-pressurizes the area behind the shell reducing the drag proportional to the amount of pressure recovered by filling the low-pressure region with gas from the base-bleed gas source. Although the base-bleed device is similar to a supplementary rocket motor with its propellant loaded interior chamber and its central flow outlet, its function is totally different from that used in shells which are fitted with supplementary rocket motors known as sustainers to increase firing range. Such rocket motors are loaded with pure rocket propellant and they provide the shell with a velocity increment, while the base-bleed device is loaded with a slow burning propellant that is intended only to eliminate drag during the portion of the shell trajectory the propellant is burning.
U.S. Pat. No. 6,657,174 describes an alternative to the base-bleed technique that involves extending the shell at the rear by a protruding conical tail section. The tail section consists of an inflatable part initially fitted in the rear section of the shell in compressed form and secured to the shell body, and can be folded out and inflated to the desired form and hardness by the propellant gases from a small propellant charge which is ignited at the required time. Such an inflatable section part can for example be made of Kevlar and remain in a removable cover connected to the shell up to the time it is deployed. The energy in the air allows the flow to turn the corner at the base of the shell following the side of the protruding conical tail reducing the area that the low pressure acts on. The drag force at the base of the shell is the difference in pressure from the outside, undisturbed air and the partial vacuum created by the separated airflow multiplied by the area that the pressure acts upon. The protruding conical tail effectively reduces the area the low pressure can act on reducing the drag force significantly. This tapered aft section is typically known as a boat-tail coming from the tapered back end of many boats designed to reduce their drag in water.